1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a structure of a cap assembly forming a top of a secondary battery and a method of fabricating the same.
2. Description of the Related Art
As is generally known in the art, secondary batteries are rechargeable and can be fabricated in a smaller size with a larger capacity than primary batteries. Secondary batteries may be classified into nickel-hydrogen (Ni-MH) batteries, lithium (Li) batteries, lithium ion (Li-ion) batteries, and polymer lithium (PLI) batteries according to the materials of the secondary batteries, or into cylinder type batteries and square type batteries according to their appearances.
According to a typical method of fabricating a secondary battery, an electrode assembly including a positive electrode plate, a negative electrode plate and a separator is seated in a can generally made from aluminum or an aluminum alloy. Electrolyte is injected into the can, and the can assembly is then sealed. Each can has an electrode terminal which is formed at an upper portion of the can and is insulated from the can. The electrode terminal has a positive or negative polarity determining a polarity of the can. In addition, each can includes a positive temperature coefficient (PTC) element, a thermal fuse, and a protection circuit module (PCM).
The secondary battery employs an electrode structure in a “jelly-roll” configuration formed by winding a positive electrode plate, a negative electrode plate, and a separator interposed between the positive electrode plate and the negative electrode plate, together. The shape of the battery determines the shape of the jelly-roll employed by the battery. That is, a cylinder type battery employs a jelly-roll wound in a cylindrical shape, and a square type battery employs a jelly-roll wound in a polygonal shape having angular corners and flat sides.
FIG. 1 is a sectional view of a conventional secondary battery having a typical construction. As shown in FIG. 1, a cylinder type secondary battery includes electrode assembly 110 for generating potential difference, cylinder-type can 120 for receiving electrode assembly 110, cap assembly 130 assembled with a top of cylinder-type can 120, so as to prevent electrode assembly 110 from being separated from cylinder-type can 120, and electrolyte 140 injected in cylinder-type can 120, so as to enable movement of ions between electrodes of electrode assembly 110. Cap assembly 130 has various safety devices provided at cap assembly 130.
Cylinder-type can 120 has clamp portion 121 bent inward so as to push cap assembly 130 inward and a bead portion 122 depressed inward so as to push cap assembly 130 upward.
Cap assembly 130 includes conductive safety vent unit 131, printed circuit board 132, PTC element 133, and positive electrode cap 134. Conductive safety vent unit 131 has a bottom welded to a positive electrode lead 111 and has a convex portion which is inverted when the battery is excessively charged or abnormally heated. Printed circuit board 132 is disposed above and is electrically and mechanically connected to conductive safety vent unit 131. Printed circuit board 132 has a circuit which is cut off when the convex portion of conductive safety vent unit 131 is inverted. PTC element 133 is disposed above and is electrically and mechanically connected to printed circuit board 132. PTC element 133 is electrically cut off when heated exceeding a predetermined temperature. Positive electrode cap 134 is disposed above and is electrically and mechanically connected to PTC element 133. Positive electrode cap 134 allows current to flow to the exterior. Insulating gasket 135 surrounds circumferential portions of conductive safety vent unit 131, current breaker 132, PTC element 133, and positive electrode cap 134, stacked on each other, and insulates them from cylinder-type can 120.
However, in cap assembly 130 of the conventional cylinder-type secondary battery, conductive safety vent unit 131, printed circuit board 132, PTC element 133, and positive electrode cap 134 are simply stacked on each other, and central portion 135a, lower dip portion 135b, and upper dip portion 135c of insulating gasket 135 are simply in contact with the circumferential portion of cap assembly 130 including conductive safety vent unit 131, printed circuit board 132, PTC element 133 and positive electrode cap 134, stacked on each other. Therefore, insulating gasket 135 cannot sufficiently seal the gap between the interior and the exterior of the can and may allow internal gas of the can to leak through a nip between cap assembly 130 and insulating gasket 135 when the internal pressure has excessively increased.